General-purpose quantum computers, if implemented in accordance with contemporary theory, would be well suited to cracking existing popular encryption algorithm such as RSA and AES.
The reason for this susceptibility is that quantum computers could theoretically test 4N possible solutions simultaneously, given N qubits (“quantum bits”) of storage. In reality, the performance would be lower, on account of “junk bits”, which are unique to quantum computing, and other overhead. Nevertheless, exponentially better performance is expected. Taken to the extreme conclusion, most industry-standard cryptosystems could be cracked in polynomial time with respect to key length.
It should be noted that at present, the largest quantum computer in use has only 128 qubits, and can only be used to solve a very narrow class of math problems.